Myoglobin Affects Tissue‐Specific Transcriptome, Heart Regeneration and Whole Animal Metabolic Rates

Myoglobin (Mb) is a small haem-containing protein traditionally associated with oxygen carrier functions in cardiac and skeletal muscle. However, studies using Mb knockout mice have yielded conflicting results regarding its functional roles in vivo. Here, we used a CRISPR-Cas-generated zebrafish Mb knockout model to investigate the consequences of Mb loss across skeletal muscle types, transcriptomics profiles, and whole-animal metabolic rates under both resting and maximal exercise conditions. Mb deficiency did not alter skeletal muscle fiber composition or overall mitochondrial respiratory capacity but induced multiple tissue-specific transcriptomic changes, including downregulation of gene sets involved in respiration and differentiation pathways in the heart, while upregulating those associated with respiration and glycogen metabolism in the skeletal muscle. During cardiac regeneration following ventricle amputation in wild-type zebrafish, Mb expression was transiently suppressed, consistent with a role in maintaining the cardiomyocytes in a differentiated state. Physiologically, Mb knockout zebrafish displayed a reduced standard metabolic rate at rest, enhanced hypoxia tolerance (i.e., a lower critical oxygen tension), and increased maximal swimming speed, while maintaining unchanged maximal metabolic rate and aerobic scope relative to wild-type counterparts. Collectively, these findings show that loss of Mb in zebrafish elicits coordinated tissue-specific transcriptional changes, potentially facilitates cardiac regeneration, lowers standard metabolic rate, and enhances maximal swimming speed and hypoxia tolerance, thereby providing new insights into the multifaceted in vivo functions of Mb.